Valuation and Qualifying Accounts - Schedule of Valuation and Qualifying Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]
Deductions from Reserves	$ 0	$ 0	$ 0
Allowance for doubtful accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Year	969	881	585
Charged to Income	136	53	773
Deductions from Reserves	(128)	(30)	(477)
Other Adjustments	0	65	0
Balance at End of Year	977	969	881
Rebates and allowances
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Year	2,297	2,303	2,164
Charged to Income	9,568	7,255	6,413
Deductions from Reserves	(8,351)	(6,809)	(6,190)
Other Adjustments	(654)	(452)	(84)
Balance at End of Year	$ 2,860	$ 2,297	$ 2,303